Sounds like an Windows Ink Services setting is causing space + click to do this, instead of the normal ArtRage behavior.
I don't see anything specific in 'Pen and Input Devices' in the control panel which immediately looks as if it would cause it however.
Can I get you to make sure you have the latest version of ArtRage ( 2.5.20 ) by looking in Help -> About ArtRage, then place a file called 'no_ink.inf' in its Resources folder to see if this makes a difference? This will stop ArtRage from using Ink Services, and may change the behavior. Any type of file will do, only the filename is important.
Also, check your Wacom Tablet Properties in the control panel, and see if setting each tab to default settings relates to this.